The glucose concentration in blood is an important marker of diabetes. Enzymatic methods which use, for example, glucose oxidase (GOD), glucose-6-phosphate dehydrogenase (G6PDH), and glucose dehydrogenase associated with pyrroloquinoline quinone as a coenzyme (PQQGDH) have been used to measure glucose concentration. Since GOD requires oxygen as an electron acceptor, however, this technique has the drawback where the level of dissolved oxygen in the analyte influences the observed data. In case of G6PDH, the coenzyme NAD(P) must be added to the reaction, which will make the detection system complicated. PQQGDH has a high glucose oxidizing activity and thus offers an advantage in that oxygen is not required as an electron acceptor. However, it has a low selectivity for glucose and exhibits a certain activity with respect to maltose as well. Accordingly, there is a need for novel enzymes which can be used as a recognition element in glucose sensors. Moreover, to enable accurate measurement of the blood sugar level in patients who is receiving xylose absorption test, it is desirable that the enzyme have a low reactivity to xylose.
It has long been known that fungi have glucose dehydrogenase (see, for example, Biochim. Biophys. Acta 139(2), p. 265-276 (1967)). The following patent documents disclose glucose dehydrogenases from Aspergillus sp. and Penicillium sp., and the measurement of glucose concentration using such glucose dehydrogenases: Japanese Patent Application Laid-open Nos. 2007-289148, 2008-178380, 2008-035748 and 2008-035747, and WO 2007/11610, WO 2004/058958, WO 2006/101239 and WO 2007/139013. However, most enzymes of fungal origin are glycoproteins, which require glycosylation for functional expression. Because most of the extracellularly secreted enzymes such as glucose oxidase are highly glycosylated, it has been exceedingly difficult to produce such fungal glycoproteins by genetic recombinant techniques in Escherichia coli. Glucose dehydrogenases of fungal origin are also extracellularly secreted glycoproteins, and thus expression of recombinant fungal glucose dehydrogenases in E. coli is difficult. Even if the enzyme is expressed in E. coli, the productivity is low, thus the yield of enzyme per unit volume of culture is extremely low.
Patent Document 1: Japanese Patent Application Laid-open No. 2007-289148
Patent Document 2: Japanese Patent Application Laid-open No. 2008-178380
Patent Document 3: Japanese Patent Application Laid-open No. 2008-035748
Patent Document 4: Japanese Patent Application Laid-open No. 2008-035747
Patent Document 5: WO 2007/11610
Patent Document 6: WO 2004/058958
Patent Document 7: WO 2006/101239
Patent Document 8: WO 2007/139013
Non-Patent Document 1: Rolke et al., Mol. Plant. Pathol. 5(1), p. 17-27 (2004)